Depression dulls sense of smell

If you're depressed, the part of your brain responsible for the sense of smell will be smaller than normal. That could explain why many psychological disorders seem to suppress the sense of smell, and it has implications for the treatment of depression itself.

Depression, schizophrenia and seasonal affective disorder all suppress the sense of smell. To find out why, researchers at the University of Dresden Medical School in Germany exposed people – 21 with major depression and 21 who weren't depressed – to a chemical with a faint odour, gradually increasing the concentration until the volunteers could smell it.

The researchers also measured the volunteers' olfactory bulbs – the part of the brain that gives us our sense of smell – using magnetic resonance imaging.

Non-depressed people were able to smell the chemical at significantly lower levels than the depressed volunteers. The depressed also had much smaller olfactory bulbs, on average by 15 per cent.

The researchers also found that the more severely depressed a person was, the smaller their olfactory bulb. The effects were present whether or not an individual was taking antidepressant drugs.

In the blood

Olfactory bulb volume and depression are probably linked by the process of neurogenesis, the growth of new neurons in the brain, the researchers say. Depression is known to inhibit neurogenesis in brain areas such as the hippocampus, and depressed people often have low blood levels of a chemical called brain-derived neurotropic factor, which promotes neurogenesis.

The work has implications for treating depression, says senior author Thomas Hummel. Olfactory bulb volume could be used as an objective measure of whether a treatment is working, he says.

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Has the flavour gone out of things? (Image: Kathrin Ziegler/Taxi/Getty)